1. Field of the Invention
The present invention relates to a liquid crystal display that needs no high frequencies to drive marginal display areas, has a simple structure, consumes little power, and achieves high response.
2. Description of Related Art
Although an NTSC system employing an aspect ratio of 4:3 is a standard television system, the development of a wide-vision system employing an aspect ratio of 16:9 has expanded the production of video software complying therewith, to allow people to enjoy video programs with more sense of realism.
There is a requirement for displays having an aspect ratio of 4:3 to cope with video programs having an aspect ratio of 16:9. For this, liquid crystal television sets and video cameras supporting the wide-vision mode have been developed.
FIG. 1 shows a display having an aspect ratio of 4:3 and supporting an aspect ratio of 16:9. Receiving a video signal of 16:9 aspect ratio, the display of FIG. 1 sets top and bottom marginal areas on a screen of 4:3 aspect ratio. Between the top and bottom marginal areas, a central area having an aspect ratio of 16:9 is secured to display an image of 16:9 aspect ratio. Without the top and bottom marginal areas, an image of 16:9 aspect ratio will be vertically expanded on the screen of FIG. 1.
According to the related art of FIG. 1, a frequency for driving the top and bottom marginal areas must be higher than a frequency used to display an image of 4:3 aspect ratio. The reason of this will be explained based on a liquid crystal display (hereinafter referred to as “LCD”) having 240 scan lines and employing the NTSC system. Driving a screen of 4:3 aspect ratio of this LCD needs 15.3 ms, which is obtained by multiplying a horizontal scan period of 63.6 μs by the number of horizontal scan lines, 240. Driving a central area of 16:9 aspect ratio secured in the screen also needs 15.3 ms.
Top and bottom marginal areas defined in the screen each include 30 scan lines. Driving the top and bottom marginal areas at the driving frequency for the 4:3 aspect ratio takes a time of 3.8 ms (=horizontal scan period 63.6 μs×60). Then, the total time for driving the top and bottom marginal areas and the central area will be 19.1 ms (=15.3 ms+3.8 ms) that exceeds one field period of 16.7 ms. Accordingly, the top and bottom marginal areas must be driven at a higher frequency.
In more detail, the 60 scan lines that form the top and bottom marginal areas must be driven within 1.4 ms (=16.7 ms−15.3 ms), and therefore, a horizontal scan period for the marginal areas must be 23.3 μs (=1.4 ms/60). Namely, when displaying an image of 16:9 aspect ratio, the driving frequency for the marginal areas must be about 2.7 times as fast as the driving frequency for the 4:3 aspect ratio. This is true not only for the NTSC system but also for a PAL system.
Increasing a driving frequency may lead to a shortage of charge in each pixel electrode of the LCD. If each pixel electrode is insufficiently charged, a black color displayed in the top and bottom marginal areas will differ in brightness from a black color displayed in the central area.
To cope with this problem, Patent Document 1 (Japanese Unexamined Patent Application Publication No. H05-199482) discloses an LCD that equalizes the potential of scan electrodes with the potential of signal electrodes in the top and bottom marginal areas. Patent Document 2 (Japanese Unexamined Patent Application Publication No. H08-314421) discloses an LCD that writes black information in scan lines of the top and bottom marginal areas.
Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2001-051643) discloses an LCD 2 shown in FIGS. 2 to 4 in which FIG. 2 is a circuit diagram showing a liquid crystal panel of the LCD 2, FIG. 3 is a view showing the liquid crystal panel and related components, and FIG. 4 is a view showing voltage waveforms in the LCD 2.
In FIG. 2, a signal line driver 11 receives a video signal whose polarity is inverted every horizontal scan period (H). A horizontal scan circuit 10 generates sampling pulses according to a control signal. In response to the sampling pulses, the signal line driver 11 sequentially supplies the video signal to signal lines X.
The LCD 2 sets an upper marginal area in a screen having an aspect ratio of 4:3 so that the remaining lower area of the screen may have an aspect ratio of 16:9 to display an image of 16:9 aspect ratio.
A wide-view control signal is at a low voltage in a period of driving the lower area, as shown in FIG. 4. In the period of driving the lower area, a switch shown in FIG. 3 is connected to a precharge pulse generator to supply a precharge pulse signal to the liquid crystal panel. In the period of driving the lower area, a precharge control signal alternates ON and OFF states as shown in FIG. 4. In the LCD 2 of FIG. 2, precharge switches PSW are set to an ON state during the ON period of the precharge control signal, to supply the precharge pulse signal to the signal lines X. When the precharge switches PSW are turned off, the signal line driver 11 supplies a video signal to the signal lines X. During a period in which the precharge pulse signal or the video signal is supplied to the signal lines X, a scan line driver 13 drives scan lines Y. Through pixel transistors Q that are made conductive with a corresponding one of the scan lines Y, the precharge pulse or the video signal is supplied to pixel electrodes P connected to the pixel transistors Q. As a result, an electric field whose strength is dependent on the amplitude of the signal is applied to a liquid crystal layer related to each of the pixel electrodes P, and the liquid crystal layer emits light whose quantity is dependent on the strength of the electric field.
The wide-view control signal is at a high voltage in a period for driving the marginal area, as shown in FIG. 4. In the period for driving the marginal area, the switch shown in FIG. 3 is connected to a wide-view pulse generator to supply a wide-view pulse signal to the liquid crystal panel. At this time, the signal line driver 11 supplies no video signal to the signal lines X, and the wide-view pulse signal is supplied through the precharge switches PSW, which are ON due to the wide-view control signal, to the signal lines X. During the period in which the wide-view pulse signal is supplied to the signal lines X, the scan line driver 13 drives the scan lines Y. Through pixel transistors Q that are made conductive with a corresponding one of the scan lines Y, the wide-view pulse signal is supplied to pixel electrodes P connected to the pixel transistors Q. Then, each corresponding liquid crystal layer emits light whose intensity is dependent on the amplitude of the signal.
To realize the two aspect ratios of 4:3 and 16:9, the LCDs disclosed in the Patent Documents 1 and 2 need additional driving systems, memories, scan converters, and the like. The LCDs of these related arts, therefore, are complicated and large and consume large power. The LCD disclosed in the Patent Document 3 must increase the amplitude of a wide-view pulse signal larger than that of a precharge pulse signal. Also, this related art must increase a current value of the wide-view pulse signal because the wide aspect ratio increases the number of pixels in a horizontal direction. This results in increasing the power consumption of a video signal processing IC shown in FIG. 3 and thus the power consumption of the LCD.
Among LCDs used for a variety of applications, those used for EVFs (electronic view finders) of liquid crystal television sets and video cameras and those used for displaying video data recorded in DVDs (digital versatile disks) require improved response to display high-quality images.
The response of an LCD may be improved by, for example, superimposing an over-drive voltage on a video signal. This, however, requires devices and line memories for computing the over-drive voltage, thereby increasing the complexity and cost of the LCD.